Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and lost cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
In one form, a guided wave radar (GWR) transmitter uses a coaxial probe that functions as an electrical transmission line into the process vessel. The GWR measurement process begins with an electrical pulse that is launched along the probe from one end. A TDR circuit identifies impedance discontinuities along the length of the probe. One source of an impedance discontinuity occurs at the vapor to liquid interface due to the difference in the relative dielectrics of the media. The TDR circuit detects, and locates in time, the reflected signal from the interface. Another source of an impedance discontinuity can be a change in geometry in the transmission line. This is a convenient method for producing a known reference location, called a fiducial (FID) in the probe. The difference in the TDR time measurements of the fiducial to the vapor to liquid interface is used to calculate the liquid level. Another impedance discontinuity exists at the end of the probe (EOP). With this type of probe and TDR circuit an increased impedance creates a positive reflected signal.
Typically, the probe includes an adaptor for mounting to a process vessel. A conductive outer sleeve, which may be part of the adaptor, receives a center conductor coaxial with the outer sleeve for conducting the pulses. Typically, a seal is provided between the outer sleeve and conductor to isolate the process environment from the outside. Under certain high pressure conditions and/or with corrosive or more dangerous materials, codes may require a secondary seal. Typically, the secondary seal is located at a near end of the probe and the primary seal is spaced a select distance therefrom. A dielectric insert fills in the space between the primary and secondary seals to maintain a continuous impedance.
Advantageously, a user will want to know if the primary seal fails. However, this may not be apparent if the failure is only to the primary seal and not the secondary seal.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.